1. Field of the Invention
The present invention relates to an organic light-emitting device, and more particularly, to an organic light-emitting device showing lower driving voltages and improved lifetime and emission efficiency.
2. Description of the Related Art
Organic light-emitting devices are active emission display devices that emit light by recombination of electrons and holes in a thin layer (hereinafter, referred to as “organic layer”) formed of a fluorescent or phosphorescent organic compound when a current is applied to the organic layer. Organic light-emitting devices have advantages such as lightness, simple constitutional elements, easy fabrication process, superior image quality, and wide viewing angles. In addition, organic light-emitting devices can achieve high color purity and perfectly create dynamic images, and have electrical properties suitable for use in portable electronic equipment due to low power consumption and low driving voltages.
Generally, organic light-emitting devices have a sequentially stacked structure of an anode, a hole transport layer, a light-emitting layer, an electron transport layer, and a cathode on a substrate. In order to facilitate the injection of electrons and holes, an electron injection layer and a hole injection layer may be further provided.
Materials that can be used to form a light-emitting layer in an organic light-emitting device are divided according to emission mechanisms into fluorescent materials using a singlet exciton and phosphorescent materials using a triplet exciton. A light-emitting layer is formed of a fluorescent or phosphorescent material alone or an appropriate host material doped with the fluorescent or phosphorescent material. Singlet excitons and triplet excitons are formed in a host during electronic excitation. At this tine, a statistical ratio of the singlet excitons to the triplet excitons is 1 to 3 [Baldo, et al., Phys. Rev, B, 1999, 60, 14422].
Organic light-emitting devices having the above-described structure are operated as follows. When voltages are applied to an anode and a cathode, holes from the anode are moved to a light-emitting layer via a hole transport layer. On the other hand, electrons from the cathode are moved to the light-emitting layer via an electron transport layer. In the light-emitting layer, the carriers are recombined to generate excitons. By the radiative decay of the excitons, light emission occurs at the wavelength corresponding to the bandgap of a light-emitting material contained in the light-emitting layer.
A hole injection layer, a hole transport layer, a light-emitting layer, an electron injection layer, and an electron transport layer constituting an organic light-emitting device are organic layers formed of organic compounds. Device deterioration occurs mainly at interfaces of the organic layers, and thus, there is room for improvement in conventional organic light-emitting devices.